1. Field of the Invention
The present invention relates to an optical head for holographic recording and to a holographic recording device including this optical head. More specifically, the invention relates to a compact optical head having integrated optical elements and to a holographic recording device including this optical head.
2. Description of the Related Art
Widely known recording devices for computers include those magnetically or optically recording and reproducing information two-dimensionally to and from a recording media. Known magnetic recording devices include Floppy® disks and hard disks. Known optical recording devices include CDs and DVDs. The recording density of these recording devices has been significantly improved in order to meet the demands for high-capacity recording media. As means for increasing the capacity even more, a recording medium applying the principle of holography has been developed.
A holographic recording device records and reproduces information in page units. The information is recorded on a recording medium as a pattern of changes in the refractive index. This pattern is created by interference of a reference beam and an object beam carrying page units of information encoded by an electronic mask known as a spatial light modulator. This pattern appears as a hologram. To reproduce the information from the recording medium, only the reference beam is emitted to the recording medium and is diffracted according to the same pattern as the hologram. In this way, the recorded information can be reproduced by a CCD. By recording and reproducing information in page units, the information can be accessed randomly and reproduced quickly. This is one of the advantages of a holographic recording device.
The holographic recording device records information on a recording medium by changing the incident angle of the reference beam to alter the interference between the object beam and the reference beam. In this way, information can be recorded overlappingly in the same space on the recording medium to increase the recording capacity. Such a holographic recording device is described in, for example, Japanese Unexamined Patent Application Publication No. 2003-43904 (corresponding US Patent Application Publication No. 2002/0191236A1).
The size of such holographic recording devices will become even smaller in the future. Currently, however, there is a technological limit to reducing the size of the optical system, in particular, reducing the size of the spatial light modulator for encoding the object beam and the CCD for reproducing the information recorded on a recording medium because relatively large units are required for producing page data.
By reducing the size of such a holographic recording device, the device may be used as a removable recording device such as a fixed disk installed inside a computer. This idea, however, has not yet been studied in detail.